Unity crashes when it tries to play a sound!

Hello, everyone

I have a problem that i don't know exactly why it is happening!

I have assigned my script to the player and then i did add audio source to the player

The problem is that, when Unity tries to play the sound it suddenly crashes!

The idea is that i want to check if the sound is playing or not, and if it is not playing then, it starts playing by fading in

Here is my code:

 void Update()

 {
     FadeInSound();

 }

 
 void FadeInSound()
 {
     float i;
     if(Input.GetKey("up"))
     {
         if(audio.isPlaying == false)
         {
             for(i = 10; i > 0; i++)
             {
             audio.volume = i * 0.1F;
             }
         }
     }
 }

Please help me

Thank you all for your time, any help is greatly appreciated.